1. Field of the Invention (Technical Field)
The present invention relates generally to the high-sensitivity detection of contaminants in gases by optical techniques generally termed photoacoustic and optoacoustic spectroscopy.
2. Background Art
Trace impurities in semiconductor process gases are among the most significant limits to product yield. Contaminants at the part-per-billion level can be problematic. In many cases, the unwanted compounds are ubiquitous in airxe2x80x94water vapor and oxygen are common examplesxe2x80x94and can enter process tools along a variety of paths. Gas suppliers and end users face two problems, guaranteeing gas purity prior to shipment and maintaining purity during distribution within semiconductor fabrication facilities. Thus, there is a need for relatively inexpensive sensors for continuous, real time measurement of gas purity. Ideal sensors would be sufficiently cost effective that one could be installed in line at each process tool. The present invention""s improvements to photoacoustic spectroscopy (PAS) and wavelength modulated photoacoustic spectroscopy (WM-PAS) provide these significant advantages for trace gas detection.
Optical spectroscopy is an effective, non-contact method for trace species detection and is well suited to continuous monitoring in process control systems. When wavelength-tunable diode lasers are used as light sources, their monochromatic output makes possible an exceptional combination of detection sensitivity and selectivity. Selectivity refers to the ability to detect the target species even in the presence of a huge excess of other compounds. Two types of techniques have been developed for achieving highly sensitive gas detection using linear optical absorption spectroscopy with diode lasers. In one case, wavelength modulation techniques (similar to frequency modulation) shift the detection bandwidth from DC, where the lasers are most noisy, to higher frequencies where laser excess noise (1/f noise) is unimportant. The other approach, called the noise canceler, uses a fast, simple transistor circuit to subtract the common mode noise in the measurements of the power exiting the laser and the power after the light beam has passed through the sample. When commercially available near-infrared diode lasers are used, both approaches have theoretical minimum detectable absorbances in the 10xe2x88x928 range for a 1 Hz bandwidth. Here, absorbance is the fractional change in laser power due to molecular absorption. In practice, however, optical artifacts in the form of unwanted interference fringes (etalons) usually limit absorbance sensitivities to xcx9c1xc3x9710xe2x88x925.
Previous work by other researchers shows that absorbances in the 10xe2x88x928 range and smaller can be detected using a simple, short (xcx9c10 cm), single pass, optical cell using photoacoustic detection. Relatively inexpensive, compact instruments for continuous monitoring of trace impurities in semiconductor process gas are possible. An improvement to PAS, called wavelength modulated photoacoustic spectroscopy (WM-PAS), eliminates a major noise source associated with traditional implementations of PAS.
WM-PAS has been practiced in the prior art. An early description of the technique was provided by C. F. Dewey, Optoacoustic Spectroscopy and Detection (Y-H Pao, ed., Academic Press, New York, 1977), pp. 62-64. Others have since practiced the technique including M. Feher, et al., Applied Optics 33, 1655 (1994); A. Miklos, et al., Applied Physics B 58, 483 (1994); and B. E. R. Olsson, et al., Applied Spectroscopy 49, 1103 (1995). All use sinusoidal wavelength modulation waveforms and do not simultaneously provide for locking the optical source wavelength to the peak of the gaseous absorption feature as with the present invention.
To reiterate, photoacoustic spectroscopy is a well-known method pioneered by Bell for measuring weak optical absorbances indirectly. Optical absorption by the target compound heats the sample. The small temperature rise creates a change in pressure that is detected with a microphone. The magnitude of the pressure change depends in part on the product of the sample absorbance and the light source intensity. Usually, the light is chopped at an audio frequency, and the photoacoustic signal is detected using a lock-in amplifier synchronized to the chopping frequency. Photoacoustic detection is useful because modern microphones have low background noise and good linearity.
Wavelength modulated photoacoustic spectroscopy eliminates a major source of noise in photoacoustic spectroscopy and provides high sensitivity detection using modest power (few milliwatt) diode lasers. Also, the use of wavelength modulation with photoacoustic detection removes the main impediment to wavelength modulated optical absorption spectroscopy, optical interference fringes. The combined techniques provide a superior method for trace gas detection.
Photoacoustic measurements are often limited by noise due to weak absorption at the cell windows. This background signal is synchronous with the chopped or pulsed laser beam and can overwhelm signals due to absorbance by the target gas. Researchers have implemented a number of approaches to avoiding window noise, such as using acoustic baffles between the windows and the microphone or trying to time-resolve the xe2x80x9ctruexe2x80x9d signal that originates closer to the microphone, but window effects remain a significant problem for photoacoustic detection.
WM-PAS avoids window noise by modulating the laser wavelength instead of the laser intensity. Optical absorption at the windows will still occur, but does not generate a synchronous, interfering signal. The basic principle of wavelength modulated photoacoustic detection is shown in FIG. 1. The laser wavelength is modulated sinusoidally across the absorption line. This wavelength modulation induces synchronous absorption which generates synchronous pressure waves at frequency f and its integer harmonics. FIG. 1 shows a photoacoustic wave whose primary frequency component is 2f. In this case, where the average (i.e., unmodulated wavelength) is coincident with the absorption line center, the 2f frequency component dominates because the laser wavelength samples the absorption line peak twice during each modulation period. These pressure wavesxe2x80x94the photoacoustic signalxe2x80x94are detected using a microphone connected to a lock-in amplifier.
The laser wavelength is modulated by a small amount: only xcx9c0.1 cmxe2x88x921 for a gas at atmospheric pressure. Absorption bands due to windows are orders of magnitude broader, so that the window absorption cross section is virtually constant across the wavelength modulation range. As a result, absorption due to the window does not introduce a synchronous acoustic signal. Wavelength modulation is ideally suited to diode lasers because laser wavelengths tune linearly with changing current. It is straightforward to add a small AC component to the laser drive current in order to effect the wavelength modulation.
The method of the present invention improves on traditional sinusoidal modulation of the wavelength in WM-PAS. It is known that the modified square wave (MSQ), FIG. 6b, modulation waveform can provide increased signal levels for 2f detection in wavelength modulated absorption experiments. T. Iguchi, Journal of the Optics Society of America B 3, 419 (1986). However, the MSQ waveform also amplifies etalon signal amplitudes in the same way as the gaseous absorption signal amplitude. Thus, in traditional WMS absorption experiments there is usually no advantage to the MSQ waveform. However, the limiting noise source in the WM-PAS technique is not usually etalons. Therefore, the MSQ waveform can provide increased PAS signals without concurrently increasing the limiting noise. The application of the MSQ to traditional WM-PAS is shown in FIG. 2. Raw photoacoustic signal is shown in FIG. 3, which has been obtained under identical conditions save for the type of wavelength modulation waveform. The MSQ waveform provides increased signal compared to the sinusoidal and triangle waveforms.
The present invention is also characterized by several additional advantages including:
Immunity to Etalons. Optical absorption spectroscopy including both wavelength modulation and noise canceler approaches are usually limited by unwanted optical interference fringes (etalons) instead of by fundamental noise sources such as laser shot noise. For commercial instruments, etalons typically constrain minimum detectable absorbances to xcx9c10xe2x88x925, which is at least two orders of magnitude worse than is predicted from shot noise alone. Etalon effects can appear only as a small perturbation on the PAS signal and not as a false absorption signal (which is the case with direct absorption spectroscopy). This advantage is realized whether the wavelength modulation waveform is sinusoidal or the MSQ waveform. Thus, this etalon immunity is maintained with the higher performance MSQ waveform.
Line locking. The combination of wavelength modulation and photoacoustic detection according to the present invention also allows a method for stabilizing the laser wavelength to be coincident with the absorption line center.
The present invention is of a wavelength modulated photoacoustic spectrometry system and method comprising: generating light from a light source; passing the light through a sample area; sampling sound produced by the light passing through the sample area with an acoustic detector; and controlling wavelength of the light with a wavelength controller, wherein the wavelength controller modulates the wavelength according to a waveform comprising square components. In the preferred embodiment, the wavelength controller additionally stabilizes the average wavelength to be coincident with an absorption line center. The waveform comprises alternating positive and negative squared peaks fluctuating about a center line, and preferably additionally comprises higher frequency and lower amplitude peaks between the squared peaks. The higher frequency and lower amplitude peaks preferably comprise a triangle waveform, a sine waveform, or a square waveform, have a frequency exceeding a frequency response of the acoustic detector, and have a frequency within the detection bandwidth of the acoustic detector with the wavelength controller employing feedback from the acoustic detector to stabilize the average wavelength to be coincident with an absorption line center. An optical detector may be employed to receive the light passed through the sample area, with its output being used by the wavelength controller to normalize the signal from the acoustic detector to light source power and/or to stabilize the average wavelength to be coincident with an absorption line center. The optical detector output is preferably demodulated at an odd harmonic of modulation frequency. A wavelength stepper may be employed with a data acquisition system for recording the demodulated signal from the acoustic detector at each wavelength step generated by the wavelength stepper. The stepping rate preferably allows at least three periods of the modulation frequency per step.
A primary advantage of the present invention is that it provides a minimum detectable concentration for moisture that is ten-fold betterxe2x80x94for the same optical path lengthxe2x80x94than the sensitivity achieved using preexisting wavelength modulated optical absorption spectroscopy techniques.
Another advantage of the present invention is that it provides measurement linearity over three orders of magnitude.
An additional advantage of the present invention is that the novel wavelength modulation waveform increases WM-PAS detection sensitivity by more than a factor of two over sinusoidal waveforms.
Yet another advantage of the present invention is that it provides combined wavelength modulated optical absorption-based optical source wavelength stabilization (line locking) with WM-PAS detection.
Still another advantage of the present invention is that immunity to etalon fringes is maintained.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.